The invention relates to a high-pressure discharge lamp comprising:
a quartz glass discharge vessel which is filled with an ionizable gas and in which electrodes are arranged opposite to each other; PA1 current-supply conductors which are connected to a respective electrode and which pass through the discharge vessel to the exterior; PA1 said discharge vessel having a first part which surrounds a discharge space, seals arranged opposite to each other and a respective neck portion of reduced width which adjoins the first part and a respective seal; PA1 said current supply conductors having in the respective seal a flat part adjoined by the glass of the seal in a vacuum-tight manner, and PA1 said electrodes comprising an electrode pin, which carries at its free end an electrode head, whose largest diameter is larger than the diameter of the electrode pin.
Such a lamp is known from U.S. Pat. No. 4,396,857.
In high-pressure discharge lamps, it may be of importance that the discharge space has a predetermined shape and size so that the lamps of one particular kind deviate only slightly from each other.
A discharge vessel is obtained from a tube, which may be cylindrical, and which is sealed at its end by causing the tube to collapse at its end or by pinching the tube at its end. For this purpose, the tube must softened at its ends by heating.
When the seals are provided, this also results in the tube being deformed between the seals. The deformation influences the shape and the size of the discharge space. It has been found that this deformation is not very reproducible. As a result, in lamps of a given kind, i.e. lamps having a small discharge space, this deformation has a great influence on the spreading of the shape and the size of the discharge space. This spreading becomes strongly manifest by the light-technical properties of lamps containing metal halide as a constituent of the ionizable gas and having a low power, for example 50 W or less.
The lamp according to the aforementioned U.S. Pat. No. 4,396,857 has neck portions of reduced width between the seals and the first part of the discharge vessel. The tube from which the discharge vessel is formed is provided with parts of reduced width before the seals are formed. They may be provided, for example, in the manufacturing step of the lamp in which the first part of the discharge vessel is formed. As a result, the shape and the size of the discharge space can be realized within very narrow limits When, at a later stage of manufacture of the lamp, the seals are formed the substantial deformation resulting from the formation of these seals is obtained at a certain distance from the first part of the discharge vessel. The neck portions of reduced width therefore constitute important means for bringing the shape and the size of the discharge space and hence the light-technical properties of lamps of a particular kind within narrow limits.
When the parts of reduced width are provided in the tube from which the lamp vessel is to be formed, in this tube two narrow ducts are formed, which, however, are sufficiently wide to allow the electrode pin with its wider electrode head to pass when the lamp is assembled.
For the properties of a high-pressure discharge lamp, the position of the electrodes in the lamp vessel, i.e. both the distance over which the electrodes project into the discharge space and their lateral position with respect to the lamp vessel, is of importance.
It has been found that the distance over which the electrodes project into the lamp vessel, is accurately reproducible, but that the lateral position of the electrodes is subject to variations from lamp to lamp.
The invention has for its object to provide a high-pressure discharge lamp of a construction which can readily be realized and which permits a small spreading in the position of the electrodes.